The present invention relates generally to an optical system for recording or replicating holograms. More specifically, the present invention relates to a hologram recording or replicating optical system capable of preventing misalignment of the center of a laser generated beam with the passage of time, which may otherwise disturb a color balance in a color hologram surface, and a multicolor hologram recording or replicating optical system which can prevent a variation of the color balance in a multicolor hologram surface, which is caused by a laser beam diameter difference for each color.
So far, R (red), G (green) and B (blue) full-color hologram has been recorded by means of an optical system as typically shown in FIG. 6 that illustrates an example of recording a full-color Lippmann hologram (reflection hologram). In this example, a photosensitive material 20 such as a photopolymer is used with light sources, an R laser 1 (e.g., 647-nm Kr laser), a G laser 2 (e.g., 576-nm dye laser) and a B laser 3 (e.g., 458-nm Ar laser). To synthesize laser beams from these lasers into one optical path, a total reflecting mirror 4 and dichroic mirrors 5 and 6 are used. In the illustrated arrangement, the dichroic mirror 5 is a red narrow-band mirror having a non-reflective coating on its back surface, and the dichroic mirror 6 is a mirror that has a non-reflective coating on its back surface and selectively reflects only light having a wavelength of 500 nm or greater. The lasers 1 to 3 are not necessarily located according to the illustrated layout, and so may be located at different positions. In this case, however, it is required to alter the positions and reflection bands of the total reflecting mirror 4 and dichroic mirrors 5 and 6.
Light coming from the RGB three-colors lasers 1 to 3 and synthesized through the total reflecting mirror 4 and dichroic mirrors 5 and 6 is split by a half-mirror 7 into two ray bundles, one of which is focused through a mirror 8 and a lens 9 to a pinhole 10. Divergent light leaving the pinhole 10 is obliquely incident on one side of the photosensitive material 20. On the other hand, the other ray bundle is focused through mirrors 11 and 12 and a lens 13 to a pinhole 14. Divergent light leaving the pinhole 14 is incident on the other side of the photosensitive material 20. Then, both divergent ray bundles interfere with each other in the photosensitive material 20, so that the hologram of an object illuminated with the divergent light leaving the pinhole 14, for instance, can be recorded therein.
For instance, to use a RGB primary-colors full-color hologram plate thereby replicating a similar hologram therefrom, such an optical system as shown in FIG. 7 is used. In the FIG. 7 embodiment, a full-color Lippmann hologram (reflection hologram) is replicated as an example. As replicating illumination light sources, an R laser 1, a G laser 2 and a B laser 3 are used as in FIG. 6. Laser light rays from these are synthesized through a total reflecting mirror 4 and dichroic mirrors 5 and 6 into one optical path. The thus synthesized RGB three-colors laser light from the lasers 1 to 3 is focused through a lens 9 to a pinhole 10. Divergent light leaving the pinhole 10 is incident on a photosensitive material 20 brought in close contact with a hologram plate 21 with an index-matching liquid filled between them. Then, the incident light and diffracted light from the hologram plate 21 interfere with each other in the photosensitive material 20, so that a color hologram having the same properties as those of the hologram plate 21 can be replicated.
When a multicolor hologram such as a full-color hologram is recorded or replicated with such a recording or replicating optical system as mentioned above, the color balance is well kept at the start of recording or replication. On completion of recording or replication, however, there is a problem that the color balance is disturbed.
Two or more laser beams are used for the recording, and replication of multicolor holograms as shown in FIGS. 6 and 7. However, the hues or tints of reconstructed images vary depending on the intensity ratio of laser light on the surface of the photosensitive material.
Although the color balance is well maintained at the start of recording or replication, it suffers from disturbance on completion of recording or replication, as already mentioned. The reason is that the distribution of each laser exposure intensity in the hologram surface varies during recording or replication, and so the ratio of each laser light intensity varies at each point in the hologram surface. The exposure intensity is strongest at the center of the optical axis of a laser beam, and becomes weak farther off the center. In other words, the change of the exposure intensity distribution in the hologram surface is tantamount to a deviation of the center of the optical axis of the laser beam from a given position. As holograms are actually recorded or replicated for a long period of time, for instance, one day or 12 hours, the center of a laser beam deviates from the center of the exposed surface. This appears to be due to the superposition of various reasons such as temperature changes.
Until now, this problem has been solved by manually measuring the exposure intensity distribution on the exposed surface, and then manually correcting the angle of each mirror in the optical system based on the measurement.
With this method, however, much time is needed for one regulation. Further, the exposure intensity distribution cannot be measured during, and simultaneously with, recording or replication.
Another problem with the recording or replication of a multicolor hologram such a full-color hologram using such a recording or replicating optical system as mentioned above is that there is a difference in the hues or tints of a reconstructed image between the central portion and the peripheral portion of the hologram surface. Such hue or tint variations on the hologram surface result from the rate of intensity decrease varying from the center to the periphery of each laser beam. Even when a hologram is recorded or replicated while the laser light of each color is balanced at the center of a photosensitive material, therefore, the balance suffers from disturbance at the periphery portion of the photosensitive material.
As already described, the fact that the rate of intensity decrease varying from the center to the periphery of each laser beam is due to a beam diameter difference between laser beams. A beam having a large diameter diverges Ad widely with the rate of intensity decrease with respect to the distance from the center becoming small. This is in contrast to a beam having a small diameter.
So far, this problem has been solved not only by making exposure intensity at the center of the photosensitive material uniform but also by measuring exposure intensity at the central and peripheral portions of the exposure surface to find the maximum intensity balance at the central and peripheral portions of the exposed surface.
With this method, however, much time is needed because several measurements should be obtained in one regulation. As the exposure surface becomes wide, there is a large hue or tint difference between the central and peripheral portions of the exposed surface. According to this method, the hues or tints thus change unavoidably more or less in the surface.
The present invention is achieved in view of such problems with the prior art as mentioned above. It is therefore one object of the invention to provide a hologram recording or replicating optical system which can make automatic correction for misalignment of the center of a laser generated beam with the passage of time, so that, for instance, the color balance in a color hologram surface can be well maintained.
Another object of the present invention is to provide a multicolor hologram recording or replicating optical system capable of preventing a color balance variation in a hologram surface, which is caused by a difference in diameter between a plurality of laser beams used upon synthesis.
According to one aspect of the invention, the above objects are achieved by the provision of an optical system for irradiating a photosensitive material with a laser beam from a laser light source to record or replicate a hologram therein, characterized in that a beam position correcting mechanism and a beam splitter are located at any position in an optical path between said laser light source and said photosensitive material and a laser beam position detector is located in an optical path split by said beam splitter, so that said beam position correcting mechanism can be operated on the basis of a beam position error signal obtained from said laser beam position detector, thereby keeping a laser beam position always constant.
In this aspect, it is preferable that in order from a laser light source side, the beam position correcting mechanism and the beam splitter are located in the optical path between the laser light source and the photosensitive material and the laser beam position detector is located in the optical path split by the beam splitter, so that the beam position correcting mechanism can be operated on the basis of the beam position error signal obtained from the laser beam position detector, thereby keeping the laser beam position always constant.
According to another aspect of the invention, there is provided a hologram recording or replicating optical system for irradiating a photosensitive material with a laser beam from a laser light source through a pinhole to record or replicate a hologram therein, characterized in that a beam position correcting mechanism and a beam splitter are located at any position in an optical path between said laser light source and said pinhole and a laser beam position detector is located at a position in an optical path split by said beam splitter and conjugate to said pinhole, so that said beam position correcting mechanism can be operated on the basis of a beam position error signal obtained from said laser beam position detector, thereby keeping a position of a laser beam incident on said pinhole always constant.
In this aspect, it is preferable that in order from a laser light source side, the beam position correcting mechanism and the beam splitter are located in the optical path between the laser light source and the pinhole and the laser beam position detector is located at a position in the optical path split by the beam splitter and conjugate to the pinhole, so that the beam position correcting mechanism can be operated on the basis of the beam position error signal obtained from the laser beam position detector, thereby keeping the position of the laser beam incident on the pinhole always constant.
In an embodiment of this hologram recording or replicating optical system where laser beams from a plurality of laser light sources are synthesized into one synthetic laser beam which is then incident on a pinhole, it is preferable that a beam position correcting mechanism, a beam splitter and a laser beam position detector are located between each laser light source and a beam synthesis optical system, so that a position of a laser beam incident from each laser light source on the pin hole can be kept always constant.
In the second aspect of the invention, the beam position correcting mechanism may comprise an angle-controllable mirror, an angle of which can be controlled around two axes intersecting at right angles and independently by means of actuators.
In the second aspect of the invention, the laser beam position detector may comprise four diodes proximately juxtaposed according to a 2xc3x972 layout, so that a detection signal from two diodes in a upper row are added to a detection signal from two diodes in a lower row to generate a vertical misalignment signal for a laser beam on the basis of a difference signal between added signals, and a detection signal from two diodes in a left column is added to a detection signal from two diodes in a right column to generate a horizontal misalignment signal for a laser beam on the basis of a difference signal between added signals.
According to yet another aspect of the invention, there is provided a multicolor hologram recording or replicating optical system using one synthetic laser beam obtained by synthesis of laser beams from a plurality of lasers, characterized in that between at least one laser and a beam synthesis optical system there is located a beam expander for converting one laser beam to make a diameter thereof substantially equal to a diameter of other laser beam synthesized therewith.
In this case, beam expanders may be located between all the lasers and the beam synthesis optical system. Alternatively, when laser beams from N (a positive integer) lasers are synthesized, beam expanders may be located between N-1 lasers and the beam synthesis optical system.
In this aspect, a beam expander having a variable beam magnification ratio may be used as at least one beam expander.
According to the first, and second inventions, the beam position correcting mechanism and beam splitter are located at any position between the laser light source and the photosensitive material and the laser beam position detector is located in an optical path split by the beam splitter, so that the beam position correcting mechanism can be operated on the basis of a beam position error signal obtained from the laser beam position detector, thereby the position of the laser beam always constant. A deviation of the center of the optical axis of the laser deviates from a given position during recording or replication, if any, can thus be automatically corrected, so that, for instance, the color balance in the hologram surface can be well maintained.
According to the third aspect of the invention, a beam expander is located between at least one laser and a beam synthesis optical system so that the diameter of the laser beam from the laser can be made substantially equal to the diameters of other laser beams. It is thus possible to make the diameters of all laser beams equal to each other so that the rates of strength decrease from the centers to the peripheries of a plurality of laser beams. When a hologram is recorded or replicated while the intensity of laser light of each color is balanced at the center of the photosensitive material, this balance can be well maintained even at the periphery of the photosensitive material. Thus, the hues or tints of the reconstructed image on the central and peripheral portions of the recorded or replicated hologram surface remain substantially identical or unchanged.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.